The present invention pertains to radial flow adsorption vessels, and in particular to radial flow adsorption vessels having a plurality of concentric permeable screens containing adsorbent particles inside a cylindrical shell.
Radial flow adsorption vessels are used in cryogenic air separation plants as front end purification systems to remove contaminants from the feed air stream prior to the feed air stream entering a cryogenic separation unit.
F. G. Kerry discussed such purification systems in an article titled "Front-Ends for Air Separation Plants-The Cold Facts," Chemical Engineering Progress, August 1991. The use of adsorption vessels in air separation plants is discussed in a paper titled "Adsorption Purification For Air Separation Units" by M. Grenier, J. Y. Lehman, P. Petit and D. V. Eyre, Cryogenic Process and Equipment, book number G00283, American Society of Mechanical Engineers (1984). Another discussion of air separation and the use of radial flow adsorbers is in an article by Dr. Ulrich von Gemmingen titled "Designs of Adsorptive Dryers in Air Separation Plants" Linde AG, Reports on Science and Technology (1994).
A major challenge in the design of radial flow adsorption vessels for use with temperature swing adsorption ("TSA") cycles is to design inexpensive, reliable containment screens that can tolerate the differential thermal expansion and contraction (caused by the cyclic temperature swings) without crushing or abrading the adsorbent particles. The challenge becomes more difficult when the temperature swings are increased and when two or more adsorbent layers are used. i There are four prevalent designs for radial flow adsorption vessels for use with TSA cycles. Each of these designs uses containment screens that are flexible in either the axial or radial direction to accommodate the differential thermal expansion and contraction caused by the large temperature swings of a TSA cycle.
U.S. Pat. No. 4,541,851 discloses a vessel which has two concentric layers of adsorbent and three permeable containment screens. The inner and outer screens are flexible in the axial direction and rigid in the radial direction. The intermediate screen is rigid in the axial direction and flexible in the radial direction. All three screens are interconnected rigidly to the vessel shell at their upper end. The lower ends of the inner and outer screens are interconnected rigidly to the lower end of the intermediate screen. The assembly of the three screens is suspended inside the vessel from the top head so that the weight of the screens and the adsorbent material is supported by the intermediate screen.
As thermal pulses move through the adsorbent bed, the screens alternately are heated and cooled. The intermediate screen expands and contracts in the axial direction and alternately squeezes and releases the adsorbent in the axial direction. The axial movement of the inner and outer screens follows that of the intermediate screen, since the inner and outer screens are flexible in the axial direction. The inner and outer screens expand and contract in the radial direction and alternately squeeze and release the adsorbent bed in the radial direction. The intermediate screen moves radially with the bed since it is flexible in the radial direction, and, as a result, imparts very little additional radial squeezing force on the adsorbent bed.
This arrangement causes the inner and outer screens to experience relatively large thermal strains (and associated thermal stresses), causes relatively large axial movement of the screens (which can result in abrasion between the adsorbent and the screens), produces relatively large axial squeezing forces on the adsorbent, and essentially limits the adsorbent bed to two layers of adsorbent.
A second embodiment disclosed in U.S. Pat. No. 4,541,851 is a vessel which has three concentric layers of adsorbent and four permeable screens. The inner and outer screens are rigid in both the axial and radial directions. The two intermediate screens are flexible in the radial direction and rigid in the axial direction. All four screens are interconnected rigidly to the shell at their lower ends. At their upper ends, all four screens are free to move in the axial direction. The three outer screens are able to slide axially in guides, while the inner most screen terminates in a dome that is able to move freely in the axial direction. This arrangement can accommodate two or more layers of adsorbent.
As thermal pulses move through the adsorbent bed, the screens alternately are heated and cooled. The design allows each of the screens to expand freely and independently of each other in the axial direction. The radial squeezing forces that are produced by differential expansion in the radial direction are transmitted to all three layers of adsorbent owing to the circumferential flexibility of the two intermediate screens.
This arrangement allows relative shearing motion between the screens and the adsorbent when the screens expand and contract in the axial direction. This shearing motion results in abrasion of the screens and attrition of the adsorbent particles. Furthermore, the guides used at the top of the three outer screens require "flexible sealing rings" to prevent the process fluid from passing through the guides. Such flexible sealing rings can be relatively expensive and unreliable.
U.S. Pat. No. 5,827,485 (Australian AU-A-57158/90, European EP-0-402-783-BI) discloses a vessel which has a single layer of adsorbent and two permeable screens, both of which are flexible in the axial direction and rigid in the radial direction. Both screens are interconnected rigidly to the shell of the vessel at their upper and lower ends.
As thermal pulses move through the adsorbent bed, the axial flexibility of the two screens allows their axial thermal expansion and contraction to be constrained by the shell of the vessel. As a result, the only axial movement that occurs is the axial movement due to the thermal expansion and contraction of the shell. Since the shell experiences very little temperature swing, this movement is very small. The radial differential thermal expansion of the screens causes radial squeezing forces on the adsorbent bed. The primary disadvantage of this design is that it is limited to a single layer of adsorbent.
German Patent No. DE-39-39-517-A1 discloses a vessel which has a single layer of adsorbent and two permeable screens, both of which are rigid in the axial and radial directions. The outer side of the inner screen is covered with a layer of permeable compressible material. The screens are interconnected rigidly to each other at their lower end. At their upper ends, the outer screen is interconnected rigidly to the vessel shell and the inner screen is interconnected to the vessel shell with an expansion joint (i.e., bellows) or it is provided with a guide that allows axial sliding. The screen assembly is suspended from the top head of the vessel, with the weight of the adsorbent and the screen assembly supported by the rigid outer screen.
As thermal pulses move through the adsorbent bed, the screens alternately are heated and cooled. The expansion joint or guide at the top of the inner screen accommodates differential axial thermal expansion and contraction between the two screens. The permeable compressible material that covers the inner screen absorbs differential radial thermal expansion and contraction between the screens to avoid large radial squeezing forces on the adsorbent bed.
There are several disadvantages with this design. First, it allows relative shearing motion between the screens and the adsorbent bed when the screens expand and contract in the axial direction. This shearing motion results in attrition of the adsorbent material and abrasion of the screens. (The permeable compressible material that covers the inner screen may be the most vulnerable to abrasion.) Second, it requires an expansion joint or a guide at the top of the inner screen, either one of which can be relatively expensive and unreliable. Third, it might be difficult to find a suitable permeable compressible material, particularly one that can withstand the relatively high temperatures of a TSA cycle. Fourth, the design does not provide for more than one layer of adsorbent.
It is desired to have a radial flow adsorption vessel for use with TSA cycles which uses two or more layers of adsorbent, and which, when compared to prior art designs, can accommodate greater temperature swings, improve the mechanical reliability of the containment screens, and reduce the attrition of the adsorbent material.
It is further desired to have a radial flow adsorption vessel wherein the thermal strain range experienced by the containment screens is significantly less than the strain range experienced by the containment screens in the prior art designs.
It is still further desired to have a radial flow adsorption vessel wherein there is less abrasion between the containment screens and the adsorbent, and lower axial squeezing forces on the adsorbent, which will result in less attrition of the adsorbent.
It is still further desired to have a radial flow adsorption vessel which can withstand the temperature swings of a TSA cycle better than the prior art vessels.
It is still further desired to have a radial flow adsorption vessel wherein there is relatively little shearing movement between the adsorbent and the containment screens.
It also is further desired to have an improved cryogenic air separation plant having an improved radial flow adsorption vessel which overcomes many of the difficulties and disadvantages of the prior art to provide better and more advantageous results.